Tryptophan, .alpha.-amino-.beta.-indolepropionic acid, is an amino acid originally isolated from enzymatic digests of proteins. Tryptophan can be obtained by extraction, fermentation or chemical synthesis. Extraction is difficult because tryptophan is easily destroyed during acid hydrolysis of proteins, and racemization occurs on alkaline hydrolysis. Fermentative methods for tryptophan production from anthranilic acid or indole have been examined. The synthetic methods are generally of two types (1) syntheses in which the alanyl residue is added to a preformed indole and (2) syntheses in which the indole portion of the molecule is formed late in the sequence. The latter route is preferable, from an industrial point of view, because the indoles are costly raw materials. Furthermore, the latter syntheses involve a minimum of conversions during which the chemically sensitive indole nucleus can be destroyed. On the other hand, the indole formation usually involves cyclization into an aromatic ring leading to mixtures of isomers. The formation of isomers is particularly acute in the case of 4- and 6-substituted tryptophans.
The instant invention provides a means for the obtention, in high yield, of racemic and optically pure tryptophans, the D-enantiomers of which have potential value as non-nutritive sweeteners.
Numerous substances have been proposed and/or used as non-nutritive sweetening agents, affording the consumer ingesting the same a sense of sweetness at least desirably comparable to that obtained with natural sugar, but without caloric effect. Such substances are necessary for some persons in order to limit intake of the natural sugars and thereby to control various health conditions, including diabetes. Many of these substances, however, have severe disadvantages. The most frequently encountered disadvantages are a bitter aftertaste and toxic side effects at rates not substantially different from those at which the sweetening effect is obtained. Only two classes of non-nutritive sweetening substances are used to any extent: saccharin-type compounds and cyclamate-type compounds. Both classes have the typical disadvantage of a bitter aftertaste; and in addition, cyclamate-type compounds have only limited activity.
Among the various categories of chemicals which have been evaluated for sweetening effect are the amino acids. A recent publication, Vuataz et al., Experientia, Vol. XXI, pages 692-694, inclusive (1965), reports the evaluation of a number of amino acids, the enantiomorphs being evaluated separately where available. The report shows that while a number of amino acids are sweet in the D form, this is not an absolute correlation. Furthermore, despite the sweet taste of the D-enantiomorph of a given amino acid, the L-enantiomorph of the same amino acid may be bitter. Resolution of the two enantiomorphs of a given amino acid is often difficult. For these various reasons, despite the contribution of Vuataz et al., no amino acid is being used in the sweetening art.
Moreover, throughout the sweetening art, it is well known that there is no correlation between structure and activity, as even relatively small changes in chemical structure often destroy activity.
In recent years the commercially-available synthetic sweeteners, saccharin and cyclamate, have encountered some difficulties in toxicological studies. Aftertaste problems have also plagued these products. It has been evident, therefore, that a need exists for new sweetening agents. This need has been met, in part, by the new sweetener 1-aspartyl-1-phenylalanine methyl ester (Aspartame.RTM.), but this dipeptide ester has shown instability under some conditions.
It has recently been discovered that certain substituted tryptophan compounds, in their D form, exhibit a sweet taste of a marked degree, at rates below rates at which any undesirable side effect may be noted. See U.S. Pat. No. 3,899,592 to Suarez et al., the disclosure of which is incorporated herein by reference.
Although it has been found that d,1-tryptophan has a sweetening capability, it is the d-enantiomorph that is the active moiety. Resolution of the d,1-mixture lessens the amount of substance needed for sweetening. Moreover, due to the fact that typically only the 1-enantiomorph of amino acids is metabolized by the mammalian body, usage of the d-enantiomorph alone, may be preferred to preclude any opportunity for the mammalian body to incorporate the substance. When, for these or any other reason, it is desired to employ only the d-enantiomorph, resolution of the racemic mixture can be achieved in procedures known in the prior art for the resolution of unsubstituted tryptophan. Three such procedures are discussed and exemplified in detail in Chemistry of the Amino Acids, Greenstein et al., vol. 3, page 2341 and following (John Wiley and Sons, Inc., New York, 1961), particular attention is directed to the first two of these (illustrative procedure 39-5 and illustrative procedure 39-6).
It is an object of the instant invention to prepare the aforementioned tryptophans either in the d,1- or the optically active form.